Note 3 - Prepaid Expenses and Other Current Assets (Details Textual) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Rio Grande LNG Site Option Lease Expense	$ 486	$ 572

The amount of lease expense for Rio Grande LNG site option.

No definition available.

Name:	next_RioGrandeLNGSiteOptionLeaseExpense
Namespace Prefix:	next_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration